Microstructure, Electrical Properties of Mn-doped Sr2-xCaxNaNb5O15 (x=0.05-0.35) Lead Free Ceramics

Lead-free piezoelectric ceramics Sr2-xCaxNaNb5O15+ywt% MnO2 (x=0.05-0.35, y=0, 0.3, 0.5) (SCNN-M) were fabricated by conventional solid-state processes, and their microstructure, phase transformation, dielectric, piezoelectric and ferroelectric properties were investigated systematically. Our results reveal that Ca2+ ions have entered into the Sr2NaNb5O15 lattices to form a solid solution and the doping of MnO2 is effective in promoting the densification of the ceramics. With the MnO2 doping, all the ceramics can be well sintered at a relative low temperature and exhibit a dense and pure tungsten-bronze structure. The addition of Ca2+ has no remarkable effect on the microstructure and MnO2 caused a significant promoted grain growth. Compared with the pure SCNN ceramics, the dielectric and piezoelectric properties of SCNN-M ceramics were greatly enhanced. At room temperature, the SCNN-M ceramics with x=0.5 and y=0.5 exhibits the optimum properties: piezoelectric coefficient d(33)=190pC/N, mechanical quality factor Q(m)=1455, planer electromechanical coupling factors K-p=13.4%, thickness electromechanical coupling factors K-t=36.5%, dielectric constant epsilon(r) =2123, loss tangent tan delta=0.038, remanent polarization P-r=4.76 mu C/cm(2), spontaneous polarization P-s=9.36 mu C/cm(2), coercive field E-c=12.68kV/cm, and curie temperature T-c=260 degrees C. Therefore, the SCNN-M ceramics are a good candidate material for lead-free piezoelectric ceramics.